Removing arena_thread_freeres suppression

It is no longer needed since we are now using a Chromium revision that
is newer than
http://src.chromium.org/viewvc/chrome?view=rev&revision=172313
In that revision, the arena_thread_freeres suppression was added to
ignore.txt.

BUG=300
TEST=tools/valgrind-webrtc/webrtc_tests.sh --tool=tsan -t
out/Release/system_wrappers_unittests
and trybot execution on linux_tsan

Review URL: https://webrtc-codereview.appspot.com/1019010

git-svn-id: http://webrtc.googlecode.com/svn/trunk@3375 4adac7df-926f-26a2-2b94-8c16560cd09d

tools/valgrind-webrtc/tsan/suppressions.txt

@@ -31,10 +31,3 @@
   fun:webrtc::Trace::Add
   ...
 }
-{
-  buganizer issue 7313086 TSAN: posix warning with 2 threads
-  ThreadSanitizer:Race
-  fun:arena_thread_freeres
-  fun:__libc_thread_freeres
-  fun:start_thread
-}